For magnetic recording media, demands toward high density recording have increased, and it has been known that the surfaces of magnetic layers can be smoothened as a means for attaining higher density recording capability.
However, when the surfaces of the magnetic layers are so smoothened, the coefficient of friction between the magnetic layer and contacted parts of the running system is increased. This results in damage to the magnetic layer of the magnetic recording media including even the separation of the magnetic layer in extreme cases after only a short period of time of usage thereof.
In order to cope with this problem of reduced running durability, lubricating agents such as fatty acids, fatty acids esters, hydrocarbons and silicone compounds have previously been used.
When the surfaces of the magnetic layers become very smooth, the amount of such lubricating agents existing on the surfaces of the magnetic layers is required to be strictly controlled. Namely, the excess lubricating agents directly lead to an increase in running friction. On the other hand, an insufficient amount of lubricating agents cause deterioration of durability. The lubricating agents have therefore been used in a manner optimizing the addition amount for each kind of lubricant thereof. In particular, the fatty acid esters improve fluid lubricating characteristics, and it is important to control the amount thereof on the surfaces of the magnetic layers.
Recently, with the popularization and expanded usage of flexible disk drive units for civilian use such as for video tape recorders (VTR), personal computers and word processors, environmental conditions under which magnetic recording media are used have been expanded. For example, there is an increased use of the media under conditions of low temperature and low humidity or under conditions of high temperature and high humidity. The magnetic recording media are nonetheless still required to be stable in running durability and storage stability, even under the severe environmental conditions under which they are expected to be used. However, the lubricating agents previously known in the art are not sufficient for such purposes.
For example, JP-A-58-218038 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-63-261526 disclose the use of fatty acid esters as lubricating agents in coating type magnetic recording media comprising non-magnetic supports provided with magnetic layers mainly composed of ferromagnetic powders and binders. However, the fatty acid esters described in these Japanese patent applications are easily hydrolyzed under the severe environmental conditions of high temperature and high humidity which, as a result, introduces the problems that storage durability is deteriorated and that sufficient running durability can not be obtained. Further, a problem is encountered in that sufficient running durability cannot be obtained under the severe environmental conditions of low temperature and low humidity.
The above-described problems become even more serious in thin metal film type magnetic recording media having thin ferromagnetic metal layers. These thin ferromagnetic metal layers are formed by vapor deposition methods such as vacuum deposition, sputtering and ion plating, or plating methods such as electroplating and electroless plating, as magnetic layers, are free of binders, and are excellent in characteristics for high density recording.
The thin metal film type magnetic recording media are also put in a high-speed motion relative to magnetic heads in the course of recording, reproduction and erasing modes. In that case, the magnetic recording media have to run smoothly and stably. At the same time, wear or breakage due to the contact with the heads needs to be avoided. However, the thin metal film type magnetic recording media have smoother, thinner magnetic layers than the above-described coating type magnetic recording media, so that the problem of providing adequate running durability becomes more significant.
As described above, in common to both the coating type magnetic recording media and the thin metal film type magnetic recording media, it has been strongly desired to develop novel lubricating agents which are both higher in storage stability and more improved in running durability over a wide range of temperature and humidity conditions as compared to the conventional lubricating agents.